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All mature forms of locomotion involve periods of unilateral stance. Unipedal hopping may provide useful information about the neuromuscular and biomechanical capabilities of a single lower extremity in adults. This study investigated whether hopping influenced vertical stiffness and lower extremity angular kinematics during human unipedal hopping. Vertical force and two-dimensional kinematics were measured in 10 healthy males hopping at three frequencies: preferred, +20%, and -20%. At +20%, compared to preferred, vertical stiffness increased 55% as hip flexion, knee flexion, and ankle dorsiflexion decreased, while at -20% vertical stiffness decreased 39.4% as hip flexion, knee flexion, and ankle dorsiflexion increased. As in bipedal hopping, the force-displacement relationship was more springlike at the preferred rate and +20% than at -20%. Given the prevalence of unilateral stance during walking, running, and skipping, findings related to unipedal hopping may be useful in the rehabilitation or conditioning of lower extremities.